actually i think this is a slim psp bug. i too had the same problem before i installed m33 on my slim. only it happens randomly so i can't figure out what causes it, but i would put psp to sleep, then sometimes when turning it back on i get a black screen for a few seconds and the psp turns off. then i turn it on again and it works.
same thing has been happening with m33, no change for me. in other words not an m33 problem.